The invention relates to fuel injectors in which an injector needle valve is urged against a valve seat by a coil spring that otherwise bears against a spring-force adjustment element positioned within a complementary bore defined in the housing opposite the valve seat.
Conventional automotive electronic fuel injectors for an internal combustion engine generally include an injector body defining an internal passage that extends between a fuel inlet and a fuel delivery nozzle. An annular electromagnetic coil assembly on the injector body encircles a portion of the passage, while an armature/needle valve assembly disposed within the passage is biased toward a valve seat by a coil spring that is also disposed within the passage. Upon energizing the electromagnetic coil assembly, a magnetic force is generated on the armature/needle valve assembly which operates against the action of the spring to move the assembly""s needle valve away from the valve seat and thereby permit pressurized fuel to flow through the injector bore and out the injector nozzle.
In order to obtain a desired spring force biasing the armature/needle valve assembly against the valve seat, the prior art generally positions the spring within the injector passage such that one end of the spring bears directly against the valve armature. The other end of the spring is typically seated against a shoulder defined within the passage, as by an end face of a cylindrical spring-force adjustment element or xe2x80x9cspring adjusterxe2x80x9d that is permanently positioned in a complementary bore defined in the injector body opposite the valve seat. The spring adjuster may comprise a solid cylindrical pin or, alternatively, may be formed of tubular stock to thereby provide a xe2x80x9cspring adjustment tubexe2x80x9d that is particularly useful, for example, in the case of a top-feed fuel injector wherein fuel flows through the spring adjuster toward the nozzle.
The prior art teaches several approaches for retaining or securing the spring adjuster at a desired depth/position within the bore in order to achieve a desired bias on the armature/needle valve assembly. Under one approach, a slightly-larger-diameter cylindrical pin is pressed axially into the bore to a desired depth. The resulting interference fit between the pin and the bore serves to thereafter retain the pin at the desired location. Unfortunately, a substantial press force is required to insert the pin into the bore, thereby increasing manufacturing costs. Moreover, the bore may be damaged during the pressing operation, creating burrs or other defects on either the pin or the bore, further increasing the pressing force required for installation and making an accurate axial positioning of the pin in the bore more difficult to achieve. The radial interference between the press-fit pin and the bore may also cause undesirable distortion of the injector body.
Under another known approach, a slotted spring pin is pressed into the bore to a desired depth. Generally, a slotted spring pin is a hollow cylindrical tube formed of thin, rolled spring steel so as to have a longitudinal slot extending down its entire length. The slotted spring pin is manufactured to a controlled outside diameter slightly greater than the inlet tube of the injector. The longitudinal slot permits the slotted spring pin to be resiliently radially compressed during installation, after which the resilient spring material of the slotted spring pin applies continuous radial pressure against the bore to maintain the slotted spring pin at the desired depth. A chamfered end on the slotted spring pin is often used to facilitate radial compression during insertion, thereby reducing possible damage to the bore and lowering the required insertion force.
Unfortunately, a significant press force is still required during insertion in order to radially compress the slotted spring pin. And, because an installed slotted spring pin does not engage the bore about its entire periphery, a greater sheet thickness must be used to achieve a sufficient resilient engagement with the bore, further increasing the press force required to radially compress the slotted spring pin and insert it in the bore, as well as the likelihood of any attendant damage to, or dimensional distortion of, the bore. Additionally, unlike a solid or tubular pin, the end of the slotted spring pin does not provide a 3600 land or circumferentially-continuous shoulder about the bore against which the coil spring may bear.
Accordingly, what is needed is a spring adjuster for an electromagnetically-actuated fuel injector that exhibits a reduced insertion force while otherwise providing both a sufficient retention force within the bore, and whose longitudinal end preferably further provides a circumferentially-continuous annular surface against which the armature return spring can bear.
Under the invention, an electromagnetically-actuated fuel injector for supplying fuel to an internal combustion engine includes an injector body defining an internal passage that extends between an inlet and a fuel delivery nozzle. An annular electromagnetic coil is mounted on the injector body so as to encircle a portion of the passage, while a needle valve disposed within the passage is biased by a coil spring toward a valve seat defined within the passage proximate to the nozzle. The needle valve is movable, upon actuation of the electromagnetic coil, between a closed position in which the needle valve sealing engages the valve seat, and an open position in which the needle valve separates from the valve seat to permit fuel to flow through the nozzle.
In accordance with the invention, a spring adjustment tube is disposed within a generally-cylindrical bore defined in the housing opposite the valve seat. By way of example, in the case of a top-feed electronic fuel injector, the bore is defined by the inner diameter of an inlet tube that otherwise also defines the upper portion of the injector""s internal passage. However, it will be appreciated that the invention contemplates forming a suitable bore in the injector body that does not form a part of a direct fuel flow path between, for example, an inlet defined on a side of the injector body and the nozzle. A coil return spring is disposed within the passage between the needle valve and an end face of the spring adjustment tube. The axial position of the spring adjustment tube within the inlet tube, relative to the valve seat, calibrates the return spring force applied to the needle valve.
Under the invention, the spring adjustment tube is formed from a square or, more preferably, rectangular section of relatively-thin sheet stock that is rolled to achieve a spiral-wound configuration having at least 1.5 turns, end-to-end, and preferably about 2.0 to 3.0 turns, end-to-end, when the spring adjustment tube is viewed in transverse section. Most preferably, the spring adjustment tube has about 2.25 turns, end-to-end, when viewed in transverse cross-section. The nominal outer diameter of the as-rolled spring adjustment tube is slightly greater than the nominal inner diameter of the injector""s inlet tube.
In accordance with an aspect of the invention, the material properties and thickness of the relatively-thin sheet stock from which the precursor square or rectangular section is obtained, as well as the minor dimension to which a rectangular section is cut, is selected such that the rolled spring adjustment tube is both resiliently radially compressible to an outer diameter at least as small as the nominal inner diameter of the injector""s inlet tube and resiliently presses against the bore upon insertion to thereby maintain the spring adjustment tube""s relative position within the inlet tube.
Because the spring adjusting tube is xe2x80x9cspiral-woundxe2x80x9d when viewed in transverse section, with the outermost xe2x80x9cturnxe2x80x9d of sheet stock overlying at least part of an innermost xe2x80x9cturn,xe2x80x9d the required resilient bending of the sheet stock during insertion is spread over a greater xe2x80x9clengthxe2x80x9d of the stock""s minor or xe2x80x9crolledxe2x80x9d dimension. Further, the innermost xe2x80x9cturnsxe2x80x9d advantageously provide a relatively-greater radial spring force for a given sheet thickness. Thus, the invention permits use of a thinner, more flexible sheet stock featuring a lower spring constant to achieve the desired resilient bias against the bore of the inlet tube, as compared to a prior art slotted spring pin, resulting in a substantial reduction in the press force required to insert the spring adjustment tube into the injector""s inlet tube.
In this regard, it is noted that, while a number of turns greater than about 3.0 may be used in connection with the invention, it is believed that the increased difficulty associated with manufacturing spiral-wound adjustment tubes having more than three turns is likely to offset the performance gains from having more than three turns and, accordingly, the number of turns employed when practicing the invention is preferably no greater than about 3.0 turns.
As a further benefit, the reduced insertion force reduces the likelihood of damage to either the spring adjustment tube or the injector""s inlet tube during insertion of the spring adjustment tube into the inlet tube, thereby facilitating precise axial placement of the spring adjustment tube relative to the valve seat for coil spring-force calibration. The more flexible sheet stock used in manufacture of the spring adjustment tube is also believed to be more tolerant of dimensional variation in the nominal inner diameter of the inlet tube, thereby permitting a reduction in manufacture costs associated with the inlet tube, while otherwise ensuring that a desired radial retention force will be obtained within the inlet tube.
In accordance with yet another aspect of the invention, where the nominal diameter or thickness of the wire stock, from which the coil spring is wound, exceeds the thickness of the sheet stock from which the spiral-wound spring adjustment tube is rolled, the end face of the spiral-wound adjustment tube advantageously defines a continuous circumferential surface against which the spring bears.